The major averages were mixed at midday Wednesday as Apple (AAPL) rebounded and Boeing (BA) sank after airlines in Japan grounded its 787 Dreamliners due to safety concerns. The Nasdaq rose 0.2%, the S&P 500 was flat and the Dow Jones industrial average dipped 0.2%. Volume was down a fraction on the Nasdaq and was 5% lower on the NYSE compared with midday Tuesday. The Nasdaq was boosted by Apple, which rose 3% in heavy buying after plunging below